Hi Muddypaws.
Just done as you suggested, my floor has a hard plastic backing and is supported ok at the sides and by the spare wheel, only problem I see would be that the spare might move about if no clamp fitted, the foam also has cut out space for the jack and other tools, which would otherwise move about. I got my inserts from Arnold Clark parts dist in Glasgow was about £50.00 with vat for both parts. Pretty sure you could order them from your local Mazda dealer as Clarks ordered them from Mazda.
Hope this is some help.